Answer:
84 students
Explanation:
Let x be the number of adults.
Then the number of children is 2x, according to the condition,
and the number of students is the rest (315-x-2x) = (315-3x).
The "money" equation (the revenue equation) is
12x + 5x(2x) + 7x(315-3x) = 2282.
12x + 10x + 7x315 - 21x = 2282
x = 2282 - 7x315 = 77 is the number of adults.
77 adults :2 x 77 = 154 children and the rest (315-77-154) = 84 students.